A working gas circulation engine, so-called a closed cycle engine, capable of circulating working gas from an exhaust side to an intake side of a combustion chamber and resupplying to the combustion chamber has been known as an engine in the conventional art. As such a typical working gas circulation engine, Patent Literature 1, for example, discloses an internal combustion engine utilizing inert gas as the working gas while combusting hydrogen as fuel in the combustion chamber. In the internal combustion engine, water vapor as a product contained in the gas exhausted from the combustion chamber is removed by a water removing device and the inert gas having water vapor removed therefrom is recirculated to the combustion chamber via a circulation route while molecules of the inert gas dissolved in the removed water is returned into a state of gas by a degasifier and resupplied to the combustion chamber as well. Accordingly, consumption amount of the working gas is reduced.